This invention relates to high molecular weight, highly linear polymers of ethylene and .alpha.-olefins, e.g., propylene, which are prepared in the presence of an aluminum-based catalyst system. More specifically, the invention relates to the synthesis of ethylene and .alpha.-olefin homopolymers and copolymers in an inert reaction medium in the presence of a catalyst system consisting essentially of an aluminum alkyl compound and an organo-Lewis acid. An advantage of the present invention is that it enables the synthesis of high molecular weight ethylene and .alpha.-olefin polymers without the need for transition metal catalysts, thereby avoiding disposal problems associated with the use of such catalysts.
It is known that the "aufbau" reaction, in which ethylene is reacted at high temperatures and high pressures to form higher olefins, occurs in two steps. In the first step, ethylene is exposed to a trialkyl aluminum compound at temperatures on the order of 90-120.degree. C. and pressures of about 100 psi to form higher aluminum alkyls. In the second step, the temperature is raised to 150.degree. C. to displace the higher alkyl groups and to form an .alpha.-olefin. While studying this reaction in the early 1950's, it was discovered that the addition to the reaction mass of earlier transition metal compounds, specifically titanium halides, resulted in the formation of high molecular polymers. Since that discovery, a variety of catalyst systems have been reported, using a variety of transition metals, including chromium (IV) oxides (Hogan, J. P., et al, U.S. Pat. No. 2,825,721), chromocenes (Karol, F. J. , et al, J. Polym. Sci., Part A, 1972, 2621), and acetylacetonate complexes of vanadium (Doi, Y., et al, Makromol. Chem., 1979, 180, 1359). Beginning in about 1980, a great deal of study was conducted in connection with highly active metallocene/methylaluminoxane (MAO) olefin polymerization catalyst systems, and more recently olefin polymerization catalysts based on diimine complexes of nickel and palladium have been reported. See, e.g., Sinn, H. and Kaminski, W., Adv. Organomet. Chem., 1980, 18, 99; Johnson, L. K. , et al, J. Am. Chem. Soc., 1995, 117, 6414; Johnson, L. K. , et al, Int. Pat. Appl. WO96/23010 (1996); and Small, B. L. , et al, J. Am. Chem. Soc., 1998, 120,4049.
For each of the known transition metal-based catalyst systems, it was believed that the transition metal played a vital role in the formation of high molecular weight polymers; and that in the absence of any transition metal, only oligomers would be produced, as in the aufbau reaction. To date, there have been few reports detailing the preparation of high molecular polymers of ethylene via transition metal-free catalyst systems. In 1992, Heinz Martin (a former student of Karl Ziegler) reported the sysnthesis of high molecular weight polyethylene by exposing ethylene to an aluminum alkyl catalyst over a period of several days (Martin, H., Makromol. Chem., 1992, 193, 1283). More recently, the synthesis of cationic aluminum complexes bearing bulky imine type ligands, as well as their potential utility as ethylene polymerization catalysts, has been investigated. See, e.g., Coles, M. P. , et al, J. Am. Chem. Soc., 1997, 119, 8125; Coles, M. P. , et al, Int. Pat. Appl. WO98/40421; Coles, M. P. , et al, Organometallics, 1997, 16, 5183; Aielts, S. L. , et al, Organometallics, 1998, 17, 3265; Coles, M. P. , et al, Organometallics, 1998, 17, 4042; Ihara, E., et al, J. Am. Chem. Soc., 1998, 120, 8277; Bruce, M., et al, J. Chem. Commun., 1998, 2523; and U.S. Pat. No. 5,777,120.
While great strides have been made in the search for new and improved ethylene and .alpha.-olefin polymerization catalysts, there remains a need for catalyst systems that are free from transition metals, that comprise only commercially available components, that require no ligand substitution, and that, nonetheless, are capable of efficiently converting monomer to high molecular weight polymer under otherwise conventional polymerization reaction conditions.